The present invention relates to electromechanical transducers, and more particularly to optical encoding devices of both linear and rotary configurations particularly adapted for providing an electrical signal output indicative of angular or linear position and/or angular or linear velocity.
Optical encoders are well known in the art and are usually classified as absolute encoders when designed to indicate position by a series of mutually unique coded output signals, or incremental encoders when capable of yielding an output signal in the form of a count or train of similar pulses.
Absolute encoders usually employ a plurality of concentric or parallel tracks each containing a number of, for example, light-opaque and transmissive elements. The tracks are typically encoded to provide a cyclic binary output, hence the number of elements doubles as one progresses from track to track. In an absolute encoder having (n) tracks, the most crowded track must therefore contain 2.sup.(n) elements. A detector is provided for reading each track. Clearly these devices are complex and expensive, especially when higher resolutions are required, and the number of tracks becomes correspondingly large.
Incremental encoders typically incorporate either a circular disc or linear strip, either of these being divided into alternative light-transmissive and opaque sectors. A variation on this basic concept of an incremental encoder and one which is well known in the art comprises a disc or strip having a plurality of tracks, each having secotrs of equivalent size, the sectors of a first track being positioned 90.degree. out of phase relative to those of a second track. The output signals of the light detectors in such an encoder are as a result also phased 90.degree., i.e. in quadrature with respect to one another, and with the addition of special phase decoding circuitry well known in the art, a directional indication can be derived from these signals. Such encoders are exemplified by the devices shown in U.S. Pat. No. 3,723,748 to R. Coburn, U.S. Pat. No. 3,912,926 to C. Coulbourn, and others.
Using the foregoing means for deriving a directional indication from an incremental encoder, it is also well known to provide up-down counter means for obtaining information on absolute position. An incremental encoder, when applied in such a manner, will obviously provide a far less expensive solution than would a similarly applied (n) track absolute encoder. One major deficiency, however, in using an incremental encoder combined with phase-decoding and counter circuitry to replace an absolute encoder is that of data loss upon a loss or failure of system power. A partial solution to this problem is taught by the current technology utilized in semiconductor memory systems where a rechargeable battery back-up system is provided to allow for data retention during a period of power loss or outage. Such a back-up system along with the use of a low power consumption semiconductor technology such as CMOS or I.sup.2 L would produce an incremental encoder-based system with the capability of retaining the position data that was present at the time that external power was removed until that power is once again restored. In the interim period, however, no position data will be obtained since power will be removed from the incremental encoder as its power consumption cannot be tolerated without resorting to excessively large batteries mainly due to the relatively large current requirements of its light emitter(s).
It is therefore evident from the foregoing description of the prior art that heretofore, it has not been feasible to provide an absolute positioning system based on a relatively inexpensive incremental optical encoder, where the acquisition of position information will be continuous, even throughout periods of temporary power outage or removal.